Cross reference is hereby made to copending and commonly assigned U.S. patent application Ser. No. 15/070,036 to Douglas K. Herrmann, filed Mar. 15, 2016 and entitled DUAL VACUUM BELT SYSTEM WITH ADJUSTABLE INTER-COPY GAP.
1. Field of the Disclosure
This disclosure relates generally to printing or recording devices that deposit ink on a recording media, such as paper. More specifically, the invention relates to such devices wherein a rotating cylindrical drum moves a sheet of paper beneath an ink jet print head. In particular, the invention relates to the aspects of rotation of the drum with recording media attached thereto by vacuum air flow while simultaneously controlling vacuum air flow through inter-copy gaps.
2. Description of Related Art
In ink jet printers where it is necessary for a vacuum drum transport to transport media under ink depositing print heads, the area where no sheet is present at the inter-copy gap creates unwanted air flow past the print heads through a gap between the print heads and media attached to the vacuum drum. This air flow creates turbulence around ink jets of the print heads and ink droplets from the ink jets are deflected from their intended trajectory. This leads to degraded print accuracy and a distorted image.
A problem is created with vacuum drum systems in that the technology used to create the vacuum under media also creates a vacuum at the inter-copy gap. With no media to block the air flow caused by the vacuum, air is pulled past ink jet heads and the air velocity causes dispersion of the jetted ink drops between the print heads and the media. This sudden change in air velocity under the print heads also induces turbulent air flow at the jetting plate surface. The turbulent air generates eddy currents across the print head face plate. These currents dry the ink meniscus in each jetting port resulting in increased numbers of misdirected and missing jets that develop during a print run. Ideally, the vacuum should be present only under the media and not at the inter-copy gap. The media, however, needs to have vacuum up to the edges of each sheet so a change in a permanent underlying plenum would create a no vacuum area under the print head and lead to the media separating from the drum and creating an uneven print surface. Hence, the need for an improvement specifically targeting those ink jet print systems with vacuum drum transport through the ink jetting zone.
Vacuum drum transports are old as evidenced by U.S. Pat. No. 8,177,231 which shows the use of a vacuum drum to move a sheet through an image forming device. The drum has vacuum, however, the system does not make any accommodations to remove air flow at the inter-copy gap or for different inter-copy gap areas of the sheet. U.S. Pat. No. 4,312,007 discloses an ink jet printer that employs a rotating cylinder to move a sheet of paper beneath an electrically driven and piezoelectrically actuated ink ejection mechanism. In U.S. Pat. No. 4,574,291 a synchronizer is disclosed for generating a pulsed trigger signal synchronized with the angular position of a rotating object, such as, a drum in an ink jet copy machine. A drive system for an ink jet printer that employs a rotating drum is disclosed in U.S. Pat. No. 4,739,346.